I do the same thing. Cathy Howse is an advocate of this method of taming the new growth when its time for a touch up so she can stretch out her relaxer time. When I reach 4 weeks post relaxer, I can not do anything with my hair except put it in a ponytail and I dont want to do that everyday. I usually relax every 6 weeks but I'm on a gradual challenge to try and stretch my relaxers taking one week at a time so this time around I will relax at 7 weeks (Jan 19th) So basically I will be straightening out my roots with the flat iron once a week after 4 weeks post relaxer, relax at 7 weeks,that means I would have used heat 3 times in 7 weeks. I really dont think thats a problem. I plan on alternating between straightening the roots after the first wash in the week and then the second wash I plan to just wear my natural texture and put it in a ponytail.Doing it this way provides versatility until I can relax.Plus I wouldnt want to put heat on my hair twice in one week. Its the ends I would be concerned with if I were you. At least with the roots,you will always be straightening new hair when its close to touch up time since new hair will always be growing out so its not like your abusing the same part of your hair over and over again.
Thanks
Tee Tee